Treatment of galvanizing skimmings



Patented Dec. 16, 1952 TREATMENT OF GALVANIZING SKIMMINGS Clyde W. Morrison, Bartlesville, Okla., assignor to National Zinc Company, Inc., New York, N. Y., a corporation of New York No Drawing. Application July 1, 1950, Serial No. 171,753

' 3 Claims.

Thisinvention relates to the treatment of galvanizing skimmings, and has for its principal object the provision of an improved process for the recovery of zinc from such skimmings.

The term galvanizing skimmings refers to the dross or slag which forms on the surface of a bath of molten zinc in which steel articles are galvanized by hot dipping. It is customary to clean the steel, prior to galvanizing, by treating it with an aqueous hydrochloric acid solution of ammonium chloride. A residue of the acid cleanin liquor adheres to the steel as it is dipped into the molten zinc galvanizing bath, and there it reacts at least in part with the zinc to form zinc chloride. This zinc chloride, together with unreacted ammonium chloride and such zinc oxide as forms spontaneously by reaction of the molten zinc with atmospheric oxygen, collects as a scum or dross at the surface of the zinc galvanizing bath. It is removed periodically by skimming, and unavoidably some metallic zinc is entrained in it.

The galvanizing skimmings thus produced comprise a complex mixture of metallic zinc, zinc chloride, ammonium chloride, and zinc oxide. The skimmings contain sufiicient zinc to make it well worth while to treat them for zinc recovery. The usual procedure for doing so is to add the skimmings to the charge fed into a conventional zinc retort. This practice is disadvantageous for two reasons: First, zinc chloride is quite volatile and most of the zinc present in the form of zinc chloride is lost. Second, the atmosphere in the neighborhood of the zinc smelter is seriously contaminated by the volatilized zinc and ammonium chlorides unless the amount of skimmings treated is held to a very small quantity per day.

It was long ago proposed to treat galvanizing skimmings with aqueous sodium carbonate solution, thereby to convert the zinc chloride to a basic zinc carbonate which could be separated from the solution and then treated in a zinc retort for recovery of the zinc as metal. For example, nearly fifty years ago it was proposed in the Meister U. S. Patent No. 714,503 to grind galvanizing skimmings and sodium carbonate together in the dry state, and to introduce the resulting ground product into water so that a double decomposition reaction could occur between the zinc chloride and the sodium carbonate. Experience with this and similar proposals, however, has been most unsatisfactory. The product resulting from the sodium carbonate treatment (even after careful filtration and washing), when treated to a zinc retort, invariably evolves large quantities of chlorides, and a considerable fraction of its zinc content is volatilized and lost; The results are so unsatisfactory as to make the sodium carbonate treatment as heretofore proposed not worth the cost of the reagents and the extra steps which its use entails.

I have discovered that if the zinc skimmings are thoroughly ground while immersed in an aqueous sodium carbonate solution,-the solid constituents of the resulting ground pulp contain all of the zinc of the skimmings but substantially no chlorides. These solid'con'stituents are easily separated from the aqueous phase of the wetground pulp, and when they are retorted in the conventional manner virtually all of the zinc of the skimmings is recovered and virtually no chloride contamination of the atmosphere occurs.

Based on this discovery, the method of the invention comprises subjecting galvanizing skimmings to a thorough grinding operation while immersed in a water solution of sodium carbonate. The solid constituents of the resulting ground pulp are separated from the aqueous component of the pulp, and are treated for the recovery therefrom of metallic zinc.

Following is a description of a preferred embodiment of the invention:

The galvanizing skimmings as received from the galvanizing plant are first crushed dry to a suitable size for charging into a ball mill or rod mill. The crushed skimmings are then delivered to such a mill, wherein they are mixed with an aqueous solution of commercial sodium carbonate (soda ash). The solution should contain at least enough sodium carbonate to be chemically equivalent to the amount of zinc chloride present in the skimmings. In a typical case, the amount of sodium carbonate present in the solution delivered to the ball mill is about A; by weight of the amount of skimmings delivered thereto; but this weight ratio of sodium carbonate to skimmings is subject to considerable variation depending on the source and character of the skimmings being treated. The sodium carbonate may be delivered to the ball mill either in the dry solid state, or already dissolved in water. In any event, sufiicient water is added to the charge in the ball mill to dissolve all of the water soluble constituents of the charge and to form a readily flowable pulp with the undissolved constituents.

The pulp in the ball mill is thoroughly ground for a substantial period of time. The retention time of the charge in the :ball mill should in general be at least in the range from one-quarter hour to one-half hour. Thorough grinding of the skimmings while immersed in an aqueous sodium carbonate solution is of primary importance to the success of the procedure.

The discharge from the ball mill is a thick pulp or slurry of solids suspended in an aqueous liquid phase. In cases where the metallic zinc content of the pulp is substantial, the ground pulp from the ball mill is delivered to a mechanical classifier for separating the coarse zinc particles from the residual slurry. The slurry is then filtered and the filter cake is washed with plain water.

The filtrate and washings contain substantially all of the chloride constituents of the skimmings, and are discarded. Substantially all of the zinc content of the skimmings is found in the washed filter cake. The latter is dried and may then be treated in the conventional manner in a zinc retort, wherein its zinc content is reduced to metallic zinc by reaction with coke (alternatively, the washed filter cake may be leached to form a solution from which the zinc may be recovered electrolytically). of dried filter cake subject to retorting at any given time need not be limited, because its treatment does not contaminate the atmosphere in any objectionable way, nor is there any unusual loss of zinc during retorting. Virtually the entire amount of zinc contained in the original skimmings is recovered in the solids separated from the ground pulp discharged from the ball mill, and substantially all of this zinc is obtained as zinc metal at the conclusion of the retort treatment. It is thus evident that the method of the invention successfully overcomes the disadvantages which heretofore have attended .the recovery of zinc from galvanizing skimmings.

I claim:

1. The method of recovering zinc from galvanizing skimmings comprising a mixture of me- The amount tallic zinc, zinc oxide and zinc chloride, which comprises subjecting the skimmings to a thorough grinding operation while immersed in a water solution of sodium carbonate, separating the solid constituents of the resulting ground pulp from the aqueous component thereof, and treating the separated solids for the recovery therefrom of metallic zinc.

2. The method of recovering zinc from galvanizing skimmings comprising a mixture of metallic zinc, zinc oxide and zinc chloride, which comprises crushing the skimmings, forming a pulp of the crushed skimmings with a water solution of sodium carbonate containing at least enough sodium carbonate to be chemically equivalent to the amount of zinc chloride present in the skimmings, subjecting the resulting pulp to a thorough grinding operation, separating the solid constituents of the ground pulp from the aqueous component thereof, and treating the separated solids for the recovery therefrom of metallic zinc.

3. In a method for recovering zinc from galvanizing skimmings comprising a mixture of metallic zinc, zinc oxide and zinc chloride, in which the skimmings are treated with a water solution of sodium carbonate, the improvement which comprises thoroughly grinding the skimmings while immersed in the sodium carbonate solution.

CLYDE W. MORRISON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 714,503 Meister Nov. 25, 1902 869,750 Terne Oct. 29, 1907 1,780,323 Waring Nov. 4, 1930 

1. THE METHOD OF RECOVERING ZINC FROM GALVANIZED SKIMMINGS COMPRISES A MIXTURE OF METALLIC ZINC, ZINC OXIDE AND ZINC CHLORIDE, WHICH COMPRISES SUBJECTING THE SKIMMINGS TO A THOROUGH, GRINDING OPERATION WHILE IMMERSED IN A WATER SOLUTION OF SODIUM CARBONATE, SEPARATING THE SOLID CONSTITUENTS OF THE RESULTING GROUND PULP FROM THE AQUEOUS COMPONENT THEREOF, AND TREATING THE SEPARATED SOLIDS FOR THE RECOVERY THEREFROM OF METALLIC ZINC. 